In the use of micromechanical sensors, self-testing concepts are frequently employed to test the micromechanical system as well as the complete signal path of the sensors. Tests based on such self-testing concepts may involve, for example, application of an electrical stimulus to the micromechanical element and measurement of the response by the sensor. The measured signal or a variable derived therefrom is compared to target values and evaluated. The target values may be documented in writing (in technical customer specification sheets, for example) or stored in the sensor itself. The self-test per se is usually composed of a sequence of stimuli, for example a positive voltage excursion, a zero excursion (no self-test stimulus), and a negative voltage excursion, or the like.
There are various ways to initiate self-tests and evaluate the corresponding sensor response to the self-test stimulus.
For certain sensors, the self-test is externally controlled by an external controller; i.e., the sensor is notified via an interface that a self-test is to be carried out. When the self-test is controlled by an external controller, the transmitted sensor response is usually evaluated in the controller.
Other sensors, in particular those which due to their unidirectional interface are not able to receive external signals, or which must be active, without intervention of a controller, very soon after the power supply is activated, automatically initiate the self-test (for example, during a start, or repeatedly during operation). When the self-test is initiated by the sensor itself, the evaluation with respect to stored target values also takes place in the sensor itself.
The sensor response to the self-test stimulus may be distorted externally or even completely overridden by interferences of the measured value. An evaluation of the sensor response with respect to stored values may then erroneously result in an unsatisfactory outcome.
The sequence of the self-test in a constant time grid results in high susceptibility to an interference signal at the corresponding frequency (the frequency of the self-test refers to the frequency of a corresponding periodic sinusoidal oscillation which best approximates a predefined periodic self-test sequence). As a result of the susceptibility, a self-test is currently dispensed with entirely for very sensitive sensors, or in installation positions with a high interference potential.
In sensors which automatically control the self-test and in which the self-test is not controlled by a controller, complex algorithms for controlling the self-test are not possible as a result of the limited computation resources, so that simple algorithms are used despite the described shortcoming.
A micromechanical sensor having error recognition is described in German Patent No. DE 10 2004 026 971, having a micromechanical function portion and an electronic evaluation circuit which are electrically connected to one another via electrical lines. This known sensor has means for carrying out a self-test of the sensor, which allow error recognition for at least one electrical connection.